From: Bill Kelly <billk@...>
Date: 2008-06-11T07:37:13+09:00
Subject: Re: Non blocking UDP


From: "Leslie Viljoen" <leslieviljoen@gmail.com>
> 
> I need to make a UDP server that waits for clients to log on and then drives
> communication from the server side. This means that if a client doesn't ack
> subsequent comms from the server, the server needs to resend those
> comms a few times before giving up.
> 
> I have gotten as far as a successful UDP server, but it blocks the thread
> forever, and having a server constantly breaking out of
> socket.recvfrom using Timeout
> does not seem to work at all (port never seems open). I need a UDP wait
> with a timeout.
> 
> I have looked at several UDP examples, and tried the sparse docs here:
> http://ruby-doc.org/stdlib/libdoc/socket/rdoc/classes/Socket.html#M004528
> 
> - but no luck yet. I tried recvfrom_nonblock as a guess but it seems to
> block anyway.

Over the years, I've never been 100% successful getting nonblock
semantics to work with UDP sockets on ruby.  I end up with code like:


require 'timeout'
require 'fcntl'

UDP_RECV_TIMEOUT = 0.5  # seconds

# ...

     begin
     timeout(0.17) {
      sock = UDPSocket.open
      if defined? Fcntl::O_NONBLOCK
        if defined? Fcntl::F_GETFL
          sock.fcntl(Fcntl::F_SETFL, sock.fcntl(Fcntl::F_GETFL) | Fcntl::O_NONBLOCK)
        else
          sock.fcntl(Fcntl::F_SETFL, Fcntl::O_NONBLOCK)
        end
      end
      n = sock.send(cmd, 0, @server_addr, @server_port)
     }
     rescue Timeout::Error
       File.open("error.log", "a") {|f| f.puts "[#{Time.now}] q2cmd: timeout in UDP open/send"}
     end

      if select([sock], nil, nil, UDP_RECV_TIMEOUT)
        begin
          # note, some linux kernel versions will select() positive for a UDP
          # packet, but the packet has a bad checksum, and when we do recvfrom()
          # the packet is thrown out, and we are blocked.  (I think, due to ruby's
          # internals, even though we're setting NONBLOCK here, doesn't help,
          # for some reason... i think this was explained on ruby-talk.)
          # Thus the 'timeout'.
          timeout(0.17) {
            resp = sock.recvfrom(65536)
          }
        rescue Timeout::Error
          $stdout.puts "q2cmd: Timeout::Error in sock.recvfrom !"
        end
      end


. . . and it still blocks occasionally, even though the socket is
in nonblocking mode.


I'm currently rewriting one of my UDP apps using eventmachine
http://rubyeventmachine.com/  ... which presumably will solve
the blocking issue.



Regards,

Bill